1. Field of the Disclosure
This disclosure generally relates to a mandrel assembly for radially expanding an opening in a structural workpiece.
2. Description of the Related Art
Aircraft and other metal structures are made up of a number of components that are typically riveted or bolted together. Drilling openings in the various structural workpieces of the aircraft is a necessary part of the assembly process and may also be done when maintaining the aircraft over its lifetime. During the aircraft assembly process, openings are necessary to attach structural workpieces together and/or to attach components to the structural workpieces. For example, openings in the structural workpiece are used to install bushings, nut plate assemblies, fastener assemblies, fittings, a rotary actuator attached to a rear wing spar, and a variety of other components. For aircraft maintenance purposes, openings in the structural workpiece may be created as a means to stop or at least impede a fatigue crack that is propagating through the structural workpiece. This latter process is commonly referred to as “stop-drilling.”
The presence of an opening in a structural workpiece creates a stress concentration in the region adjacent to the opening. Under cyclic tensile loads, the opening acts as a stress riser and is often the initiation site of a fatigue crack. In the case of stop drilling, the opening is drilled at the tip of the fatigue crack to at least temporarily impede the growth of the crack. Once initiated, a fatigue crack may propagate to a critical length, thus resulting in a fatigue failure of the structural workpiece and/or the detachment of a component attached thereto.
Whether attaching components to the structural workpiece or stop drilling, for example, the formation of the opening is typically accompanied by some form of cold expansion process to create a beneficial state of residual compressive stress around the opening. The cold expansion process usually employs a tapered mandrel forcibly drawn through the opening to cause a radial plastic flow of material in an annular zone around the opening. This plastic flow of material results in the residual compressive stresses and may extend up to one diameter beyond the edge of the opening. In one example, the process of installing a bushing includes pulling the tapered mandrel through the bushing to radially, plastically expand the bushing into the opening of the workpiece. This type of cold expansion process can also create an annular zone of residual compressive stress in the structural workpiece surrounding the bushing.
The cold expansion process using the tapered mandrel and an associated split sleeve is described in U.S. Pat. No. 3,566,662; U.S. Pat. No. 3,892,121; U.S. Pat. No. 4,471,643, and U.S. Pat. No. 5,083,363. In the foregoing references, the tapered mandrel includes an elongated rod having a first end region and a second end region. The first end region is generally fixed to a puller/installation tool, usually by a threaded engagement. The second end comprises an increasing diameter conical portion that is sized to closely pass through the opening when the second end is inserted therein. The split sleeve is placed on elongated portion of the rod, which is smaller in diameter than the second end. The split sleeve is moved into the opening after the second end of the mandrel has been inserted through the opening.
The inner diameter of the split sleeve is selected to be smaller than the second end of the mandrel so that, when the second end is pulled back through the opening, the second end radially expands the split sleeve into the opening and may indirectly expand the surrounding material of the structural workpiece. The split sleeve undergoes significant plastic deformation in order to effectively expand the material of the structural workpiece. The disposable split sleeve generally operates to reduce the mandrel pull force and shield the opening from contact frictional forces generated by the high interference of the expansion mandrel as it is inserted and/or extracted from the opening. In addition, the split sleeve may be available in a variety of sizes to achieve a proper radial expansion of the opening. Because the split sleeve is permanently deformed a substantial amount during extraction of the mandrel from the opening, the split sleeve is rendered unusable for further cold expansion operations.
One of the primary goals of the aforementioned mandrels is to provide a means to cold expand the opening in the structural workpiece from only one side of the workpiece. However, one of the drawbacks of using the split sleeve cold expansion process described above is that the split sleeve is a disposable part and is useable for only one operation. Since there are typically thousands of holes in the assembly of a commercial vehicle such as an airliner, the assembly plant must maintain a large inventory of split sleeves.
This problem is partially addressed in U.S. Pat. No. 4,557,033, which provides a mandrel that can be removably attached to the installation tool. However, the mandrel must be moved through the opening of the structural workpiece from the blind side of the workpiece. Hence, both sides of the workpiece must be accessible to use the removably attachable mandrel disclosed in the '033 patent.
Based on the foregoing, there is a need for a mandrel assembly that is removably attachable to an installation tool and that allows cold expansion of the opening from only an access side of the structural workpiece.